In mobile communications systems, the goal is that the mobile station connects to the rest of the telecommunications network via the base transceiver station that offers the best transmission path. Communications between the mobile station and base transceiver station takes place over a radio path. Data transmission from the base transceiver station to the mobile station is known as downlink or forward link transmission, and data transmission from the mobile station to the base transceiver station as uplink or reverse link transmission.
Traffic over radio links is subject to a wide variety of transmission losses and interference. When the distance to the transmitter increases, signal strength in free space decreases by the square of the distance. Viewed from the base transceiver station, a mobile station may also be blocked by a building or hill, which will attenuate the signal. Often, the signal is attenuated by signal reflections. In such a case, the mobile station receives the same signal from several directions, for example due to reflections from buildings. Such reflections either weaken or amplify one another. FIG. 1 illustrates signal attenuation caused by distance, geographical obstructions and reflections. The horizontal axis represents the distance between the base transceiver station and the mobile station and the vertical axis the strength of the signal. The solid line shows signal attenuation in free space, the dotted line the impact of geographical obstructions and expanded detail the effect of reflections (multipath propagation) on the signal. The horizontal dashed line denotes the minimum sensitivity of the assumed mobile station at which it is still capable of detecting the signal.
Similarly, a mobile station that is moving, and/or moving reflective surfaces in its vicinity, attenuate the signal due to the Doppler effect. When the speed at which a mobile station is travelling increases, the level of attenuation per unit of time caused by multipath propagation increases as well. In digital mobile communications systems, reflections and the Doppler effect also cause a phenomenon known as time dispersion, which complicates the identification of a symbol (such as the bit status 0 or 1) arriving via multiple routes.
In mobile communications systems, transmission power is regulated by the mobile station and/or base transceiver station in order to reduce the level of interference in the network and to compensate for fading on the radio path. The idea with power control is to maintain the received signal at a constant yet as low as possible level while at the same ensuring that the quality of the received signal remains optimum. When the level and/or quality of the signal on the radio path between the mobile communications network and the mobile station falls below the preferred standard, transmission power can be adjusted by the base transceiver station and/or mobile station to improve the quality of the radio path. As a rule, transmission power is regulated from the fixed network using a special power control algorithm. The mobile station monitors the strength and quality of the downlink signal from the base transceiver station whereas the active base transceiver station monitors the strength and quality of the uplink signal from the mobile station. Using the results of the measurements and the pre-set power control parameters, the power control algorithm defines the suitable transmission power level that is then transmitted to the mobile station in the form of a power control command. Power control is carried out on an ongoing basis while the call is in progress. In known TDMA mobile communications systems, such as the GSM system, this takes typically place twice per second. Increasing the transmission power increases the level of interference in the network, which is why transmission power levels are kept as low as possible. Transmission power control also offers savings in the mobile station's power consumption.
To offset the effects of fading, i.e. losses and interference, mobile communications systems use methods such as frequency hopping and/or antenna hopping. The efficiency of frequency hopping in counteracting fading is based on frequency dependence. In antenna hopping, the current signal transmission path is replaced by a new path, which alters the fading affecting the signal.
Frequency hopping can be used to reduce co-channel interference caused by signals from several base transceiver stations and the effect of radio path fading on the signal to be transmitted. This is accomplished by changing the frequency used on the radio path according to a predefined frequency hopping pattern. Frequency hopping can be achieved by using carrier frequency hopping or by changing the radio frequency specifically to individual transmitters. Normally, hopping is performed in the cycle of a single burst (time slot).
Another property of the signal to be transmitted that can be changed is its frequency deviation. For example, in a dual antenna array, transmission from one of the antennas may include a minor frequency deviation to simulate fast fading.
Another way of diminishing the impact of fading on the signal to be transmitted is to use antenna hopping, where the signal is transmitted and/or received alternatingly by two or more antennas that are physically separated. Then, the propagation path of the signal to the different antennas is different. Because fading is not only frequency-dependent but also location-dependent, changing the propagation path may improve the propagation conditions. In antenna hopping, the transmitting and/or receiving antenna is changed according to a predefined hopping pattern. Another feature closely associated with antenna hopping is that it is possible to modify or replace the current form and strength of antenna beams. It should be pointed out that antenna/frequency hopping is relatively ineffective against noise, meaning that hopping must be accompanied by error correction, such as channel coding.
It is also possible to use delay diversity at the transmitting end. If, for instance, transmission takes place simultaneously via two antennas, transmission from one of the antennas can be delayed and the duration or phase of the delay, or both, altered.
Error correction, such as channel coding and/or retransmission, and bit interlacing can be used to improve the quality of the transmission and the transmission error tolerance. In channel coding, redundant data is inserted in the data to be transmitted, which makes it possible to detect the original data correctly even if errors were induced in the signal on its transmission path. Retransmission is used for transmission error correction either independently or in addition to channel coding, in which case errors in channel-coded transmission are corrected by retransmitting the distorted frames. Interlacing the bits to be transmitted means that bits of several code words are intermixed, making the bits adjacent to the signal disperse into several bursts. Thanks to interlacing, the signal can normally be detected even if an entire burst were lost in transmission.
The purpose of the present invention is compensate for fading by means of suitable transmission diversity. One known method based on transmission diversity is presented in patent application EP-741 465. In this publication, the mobile station selects the best of the signals from several transmitting antennas and communicates this selection to the base transceiver station that continues to transmit via this particular antenna. The base transceiver station inserts in the first data packet its identifier and sends the first data packet with its identifier via one antenna. Similarly, the base transceiver station inserts in the second data packet its identifier and sends the second data packet with its identifier via another antenna. The mobile station receives both transmissions and compares the received signal levels with each other. After selecting the optimum transmission branch, the mobile station communicates the identifier of the selected packet to the base transceiver station in a control time slot. The base transceiver station will transmit to the mobile station involved via the antenna indicated by the same. According to the publication, signals from several subscriber connections are intermixed in connection with coding before the transmission is routed to the various transmission branches, meaning that all the user data will be sent via the same transmission antenna branch using the transmission diversity disclosed in the publication. The method explained in the publication is also suitable for situations where transmission and reception frequencies differ from each other, i.e. when frequency division duplex FDD is used.
One way of evaluating known techniques is to divide them into systems that use feedback and systems that do not. In feedback systems, the receiver sends measurement data to the transmitter that selects the transmission diversity based on the measurement data. Feedback provides measurement data on the status of the channel, and so these systems perform better than systems that do not make use of feedback and carry out antenna hopping irrespective of whether there is any fading on the channel or not. The drawback of systems using feedback is that the measurement data requires a return path (capacity in the uplink data transmission frame) and that they are more complex than systems that do not make use of feedback data. For example, the antenna hopping pattern described above is part of a system that does not use feedback. By contrast, EP-741 465 is a solution that relies on feedback.
The purpose of the invention is to provide the advantage of feedback when using transmission diversity without taking up any additional capacity in the uplink data transmission frame to indicate the measurement data or selection/change of the preferred diversity option.